I called a couple schools about this and I'll tell you how I interpreted the responses. If you have three strong letters that cover all the bases, then you shouldn't submit a fourth. If there is another aspect to your education or experience that is not covered, a fourth strong letter can help.
Personally, I was trying to decide who to ask for my third letter--someone who knew my classwork or someone I had never had a class with but who knew my research (not my boss at my research internship, that was a definite). They told me that after the first two years, students just do research so they would be more interested in reading a letter from someone who could talk about my research potential.
Of course, being a theorist, that sounds like a strong letter. Really, I'd call up a couple departments and guage their response.